"If we've learned anything in the last 10 years, it's how to kill a coral reef," says coral ecologist Terry Hughes ruefully.

Those dire facts, drawn from the latest "Global Coral Reef Status Report," however, are serving as a springboard for devising strategies to save the world's coral communities and, by extension, the thousands of marine species that rely on them. The best way to do this, many marine ecologists now maintain, is to focus on a reef's ability to bounce back from hardship. Where ecologists once talked about saving species, habitats, and biodiversity in a tropical reef ecosystem, many now speak of preserving "resilience."

Nowhere is this approach to reef conservation being put through its paces more rigorously than along Australia's Great Barrier Reef - dubbed by some the largest living thing on Earth. In fact, the GBR is a chain of 2,900 reefs stretching some 1,200 miles along Australia's east coast. Slowly expanding its reach as sea levels have risen following the last ice age, the network covers just over 135,000 square miles of coastal ocean.

Beyond its immediate biological value, the reef system represents a ringing cash register for the state of Queensland. Tourism and related activities bring in roughly $1.2 billion (Australian; US$950 million) a year to the region. The reef network also serves as a buffer between the mainland and the high seas that accompany tropical cyclones.

In the past, researchers would study tropical-reef response to single events - such as a hurricane, tropical cyclone, or coral bleaching - to evaluate its ability to bounce back.

"People wrote about these as one-off events," explains Dr. Hughes, a professor at James Cook University in Townsville. "But on longer time frames - from decades to centuries - those are recurrent events. We're now asking: How can this system, on a scale of thousands of kilometers, absorb recurring disturbances without going belly-up? Resilience is about the system absorbing changes" and conservation managers "being proactive in anticipating them."

Calls for this broader approach have been heard for some time. But the need was driven home by virtually back-to-back coral-bleaching events in the summers of 1998 and 2002, during which coastal waters grew unusually warm. Under conditions of high heat and light, the algae that lived in the coral, provided it with food, and gave it its distinctive color underwent a Jekyll-and-Hyde change. The algae became corrosive, eating away at the coral from the inside. In self-defense, the coral expelled the algae - and with it the coral's source of food. The coral turned white and died.

Both bleaching events involved vast tracts of coral, with 2002's event marked as the worst bleaching event on record along the GBR. To many scientists here, these were harbingers of the future as Earth's climate warms - at least in part because of carbon dioxide rising into the atmosphere as humans burn coal, oil, and other fossil fuels.

Even modest sea-surface warming, say 1 or 2 degrees C - the midrange forecast of the Intergovernmental Panel on Climate Change - could prompt large declines in coral communities by 2050, according to Ray Berkelmans and colleagues at the Australian Institute of Marine Science (AIMS) and the US National Oceanic and Atmospheric Administration in a study last year.

The concern: As global warming heats the ocean surface, bleaching events will happen more often, giving reefs less time to recover - all other things being equal, Dr. Berkelmans and others say. Added to that stress, they say, is the destruction that would come from tropical cyclones, which are expected to grow more intense, if not more frequent, as global warming proceeds.

One potential offset to bleaching could be coral's potential for adapting to warmer waters. Some coral communities have been able to survive warmer waters by embracing algae that are more tolerant of heat than their previous tenants, according to a team of scientists led by Andrew Baker, a researcher with the Center for Environmental Research and Conservation at Columbia University in New York. Their findings were reported in the journal Nature last August.

But there may be limits to how broadly these results apply, others say. Even without pressure from climate change, reefs also face pressure from overfishing, farm and ranch runoff, and soil erosion.

Last July, after several years of debate and negotiation, Australia's federal government took a significant step by declaring fully one-third of the reef a no-take zone - no fishing, capturing live fish, or collecting corals. Previously, no-take areas covered only about 5 percent of the reef.

At the same time, the state of Queensland adopted a program for reducing the silt and nutrients that flow onto the reef from rivers in the region. The silt can cut light and smother young coral before they can replenish a bleached area. The nutrients can lead to explosions of algae and Crown of Thorns starfish, which can turn healthy reefs into drab undersea barrens.

Having figured out the historical impact of silt from rivers (see story, below), an AIMS team is embarking on a five-year project to pin down more precisely the biological effects of the nutrients and soil across broader reaches of the reef system.

As these scientists head to the reef to get a better handle on the factors that determine the GBR's resilience, others are using those data to build models to forecast resilience.

For example, Scott Wooldridge is developing a "state of the reef" computer model at AIMS that will allow conservation managers to rank the resilience potential for different reefs or reef segments. The model has the potential for use worldwide. So far, he's included three elements: adequate levels of grazing fish on the reef to keep algae at bay, water quality, and increased heat- tolerance among coral - which he acknowledges is the weakest link in the chain in terms of biological research.

The model points to some disturbing results. Australia - and specifically, the Great Barrier Reef Marine Park Authority - may have chosen the wrong approach when it set up its no-take areas, he says.

His preliminary results suggest that the northern third of the reef probably should get the most conservation attention. The park agency, by contrast, set aside ecologically representative areas scattered throughout the reef. That made sense at the time, Dr. Wooldridge says, given what scientists then knew. But the northern segment is more pristine and faces fewer stresses because fewer people live and visit there. While it will likely feel the bleaching effects of climate change more strongly at first than reef sections farther south, it still stands a good chance of surviving. Thus it will be able to provide the larvae that will ride prevailing currents south to reseed portions of the reef that are under greater multiple stresses.

It's a controversial notion, Wooldridge acknowledges, and calls into question the strategy over which the government spent so much time and political capital.

"With proper management, you can still have a viable reef by 2050," he says. "But the implications are that we need to conserve more in the north."

When scientists play history sleuth

Canberra, Australia

To persuade people to protect the environment, scientists' future projections can fall short. Often they need to tease out clues from the past.

As Queensland was debating whether to protect the Great Barrier Reef by restricting river runoff, coral researchers could point to historical records - not to mention contemporary recollections - that the state's rivers had grown more silt-filled since European settlement, threatening the reef. But they had very little data to back it up. "We knew things were happening," says Malcolm McCulloch, a geochemist here at Australia National University. "But we didn't know the true scale of what went out to the reef."

In what many here see as a seminal piece of sleuthing, Dr. McCulloch and colleagues from the Australian Institute of Marine Science took core samples of coral, which develop annual growth bands like trees. Looking for chemical signatures of soil run-off, particularly the element barium, they found that from 1750 to about 1870, sediments from the Burdekin River - the country's second largest when it floods - reached the inner portions of the reef "only occasionally." After about 1870, the amount of soil disgorged to the inner reef grew five- to 10-fold as land upriver was cleared for ranching and grazing began.